In contrast to the conventional light emitting devices such as incandescent lamps or fluorescent lamps, cold cathode field emission type light emitting devices have recently been developed which call for electrons emitted in a vacuum from a cold cathode electron emission source to be thrown at high speed upon a light emitter, thereby exciting the light emitter to emit light, providing the anticipated prospect of its use for field emission lamps (FELs) or field emission display devices (FEDs).
Among these light emitting devices, in particular, FELs, which are specialized for use in lamp light sources and the like can be fabricated with a simple construction in which cathode electrodes, anode electrodes and the like are placed in a vacuum chamber having glass substrates mounted oppositely to each other, with a predetermined spacing. In this case, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-339012, the anode electrode is generally constructed by film-forming a transparent electrically conductive film such as ITO (indium-tin-oxide) or the like on the back side of a glass substrate which forms a light projecting plane on a vacuum chamber, and on a top layer of the anode electrode is film-formed a light emitter.
It is noted that vacuum chambers, in this type of light emitting devices, are fabricated via a vacuum sealing step at high temperature. Therefore, in order to prevent the generation of cracks and the like by thermal expansion during the vacuum sealing step and the like, generally the vacuum chambers are constructed through a combination of glass base materials with equal coefficients of linear expansion. In this case, although it is also possible to use quartz glass or the like as a glass base material, it is preferable to use soda-lime glass which is easy to process and moreover available at low cost in the market so as to hold down the manufacturing costs.
On the other hand, soda-lime glass or the like, lacking in heat resistance, tends to fracture with a local temperature rise due to thermal distortion. It is necessary herein to control the current density between the cathode and anode at a high level for a light emitter to emit high brightness light; however, as is well known, the higher the current density, the higher is the temperature of the light emitter. For example, the light emitter undergoes a temperature rise of about 30 degrees C. with an increase in the luminescence intensity by 0.1 W/cm2. Therefore, light emitting devices in which the vacuum chamber is fabricated of soda-lime glass or the like have been limited in causing the light emitter to emit high brightness light.
The present invention has been made in view of the above-mentioned situation, and it is an object thereof to provide a light emitting device capable of providing both improved durability with an inexpensive and simple construction and higher brightness.